The present invention relates generally to electronic and electromagnetic devices for the determination of the presence and strength of selected substances which include ions, molecules and ligands.
Often times it is advantageous to detect the existence and/or concentration of substances in the atmosphere or solutions. Generally such detectors involved exposure of a chemical compound to the test medium with the indication of existence and/or concentration being provided by a color change, the formation of a precipitate, etc.
In the recent past, however, it has become possible to provide semiconductors which are sensitive to the exsistence of specific substances in liquids or gases, and provide an electrical indication of both the presence and concentration therein. Examples of these are the ion-sensitive field effect transistor (ISFET), one example of which is shown in U.S. Pat. No. 3,831,432, and the ion controlled diode disclosed in co-pending U.S. patent application Ser. No. 781,474, filed Mar. 25, 1977, herein incorporated by reference. Both devices incorporate an ion sensitive membrane which forms a charge layer near the semiconductor junction which is dependent upon the concentration of the ion, molecule or complex, whose concentration is to be determined. The substance-sensitive material varies widely in its individual makeup dependent upon the particular substance to which the material is sensitive. Many different materials can be used in the membrane construction in order to achieve varied substance-sensitivity. For example, one such substance-sensitive material which is sensitive to potassium ions (K.sup.+ ions) is valinomycin. Some other substance-sensitive materials are listed in Membrane Electrodes by N. Lakshminarayanaiah, Academic Press, 1976, also herein incorporated by reference.
In the prior art substance-sensitive devices, the substance-sensitive material is formed into a cast membrane which is then located over the semiconductor device. Alternately, the semiconductor device can be coated with the appropriate material. While these methods of forming the substance-sensitive membrane are suitable for a single device capable of providing an electrical indication of the concentration of a single complex, it is extremely difficult to utilize cast membranes, or substance-sensitive coatings, to render only discrete portions of a Large-Scale Integrated (LSI) circuit involving semiconductors, microwave striplines or integrated optical structures sensitive to the substance. Thus, in the common mass production of integrated circuits, it is extremely difficult, if not impossible, to provide appropriate amplifier circuitry along with a substance-sensor in a mass-produced integrated semiconductor, microwave or optical circuit.
The difficulty of obtaining precise placement (measured in microns) of a cast membrane, eliminates the applicability of cast membranes to LSI structures. If existing LSI structures are coated with substance-sensitive materials, it would be possible to detect only one substance, and not a plurality of different substances.